Exemplary embodiments disclosed herein are directed to tire pressure monitoring systems for vehicles that can also include keyless entry systems. Many cars today apply SMART entry by employing 125 KHz low frequency (“LF”) antennas in many locations around the vehicle to localize a SMART search field. For example, an LF antenna can mount to a driver door handle to search an area surrounding the driver door for a portable transmission/reception unit, e.g. a key fob, for keyless entry to the vehicle. These LF SMART search fields are highly controlled, such that specific search patterns can be realized.
By regulation, all vehicles in the United States must include a tire pressure monitoring system (“TPMS”). In a known TPMS, an LF antenna is mounted in each wheel well such that a TPMS control unit can trigger, or wake up, each TPMS sensor, which are each located in a respective tire, via the LF antenna. The TPMS control unit sends a signal to the LF antenna to transmit an LF field to wake up the TPMS sensor. The TPMS control unit can receive a near instantaneous response such that the tire pressure for each wheel is known in seconds and can be presented on a display in the vehicle cabin for the vehicle operator.
Like the known SMART entry systems, known TPMS employ 125 KHz LF antennas. For a vehicle including both SMART entry and TPMS, up to ten individual LF antennas can be required on the vehicle. Both the TPMS sensors and the SMART entry fobs reply to a receiver in the vehicle with an RF signal around 315 MHz. Both systems are slightly offset from this frequency for independent operation, but the frequencies are close enough that similar antennas can be used to receive the signal.
Combining SMART entry and TPMS to reduce the number of LF antennas on the vehicle can result in problems. Known combined SMART entry and TPMS may fail to provide tire pressure data to the operator at the moment (or nearly instantaneously after) the ignition for the vehicle is turned ON. This is due to the fact that localization of the tire sensors can be difficult when the antenna that is used to wake up each tire sensor is not mounted within a respective wheel well. Additionally, since both the tire sensors and the fobs wake up upon detecting an LF field, the fob may respond by sending a signal to a receiver in the vehicle when the LF field was intended to only wake up the tire pressure sensors, and vice versa, This can result in an undesirable drain on the power source for the tire pressure sensors and the fob.
Moreover, providing tire pressure data at ignition ON can present issues. Waking up a tire sensor without moving the tire can present a problem because the tire sensor can be located at nearly an infinite number of positions when a vehicle is parked, and some of these positions may not be conducive for detecting an LF field generated from an antenna that is not located within the wheel well. Moreover, transmitting an LF field strong enough to wake up tire sensors may result in a broadcast AM receiver, which is typically connected with a vehicle radio, picking up the LF field such that undesirable sound is emitted from a speaker connected with the radio. Moreover, problems may arise localizing one or more of the tire sensors when one of the SMART entry and TPMS antennas is mounted on a vehicle door and the door-mounted antenna emits an LF search field to wake up the tire sensors. Moreover, recognizing tire sensor locations when in the manufacturing facility can also present issues.
One known TPMS sensor available from TRW Automotive includes two LF coils transversely mounted on a printed circuit board (“PCB”) used for reception of LF commands from an on-vehicle mounted antenna, which is mounted in the wheel well, or from an external device such as registration equipment or a service tool at dealerships. The orientation of the LF coils on the PCB is shown in FIG. 11. The vertically-oriented coil 2 in FIG. 11 is used for normal LF reception from the vehicle mounted antennas, which is believed to be about 99% of its use. The second horizontally-oriented coil 4 in FIG. 11 does have some benefit for static LF transmissions as well, which is most important for the auto-learn sequences; however, the second coil 4 is not optimized for LF reception from the vehicle mounted antennas.